Autocomplete扩展器的Web服务不工作

我使用了ajax自动扩展程序，代码就像一样

 <asp:ToolkitScriptManager ID="ToolkitScriptManager1" runat="server">
    </asp:ToolkitScriptManager>
    <div>
    <asp:TextBox ID="txt_AutoComplete" runat="server" Width="200"></asp:TextBox>
    <asp:AutoCompleteExtender ID="AutoCompleteExtender1" runat="server" MinimumPrefixLength="1" 
    ServiceMethod="GetNames" ServicePath="~/AutoComplete.asmx" TargetControlID="txt_AutoComplete"> </asp:AutoCompleteExtender>

在Autocomplete.asmx中，web服务没有调用

[WebMethod]
        public string[] GetNames(string prefixText, int count)
        {
            ArrayList sampleList = new ArrayList();
            sampleList.Add("ABC"); sampleList.Add("Hello");
            sampleList.Add("Hi");
            sampleList.Add("Hey");
            ArrayList filteredList = new ArrayList();
            foreach (string s in sampleList)
            {
                if (s.ToLower().StartsWith(prefixText.ToLower()))
                    filteredList.Add(s);
            }
            return (string[])filteredList.ToArray(typeof(string));
        }

但是ajax自动扩展程序不工作